How Do I Get Back At My Sister Who Went On My Computer?

[Banana]

I just found out that my 16 year-old sister went on my computer at about 3PM yesterday and fucked with all of my stuff. She went on my facebook, my twitter, my spotify, but what pissed me off the most is that she read all of the messages that I've sent/been sent from/to girls. She always goes on my facebook if I leave it for 5 minutes at my dad's, and I've had enough.

I want to seriously get back at her. Enough so that she doesn't do it again.

On my cousin's birthday we had a party, and whilst my cousin was out buying food for the party, we turned everything in his room upside-down. I thought this was an amazing prank and I think I might try it.

Although, it's a lot of work for one person, and I want to know if anyone has something better. Help!